Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. In order to produce ethylene oxide, ethylene and oxygen are passed over a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent (absorbent) stream containing mostly water. After absorption, the aqueous ethylene oxide stream is sent to a stripper in order to separate the ethylene oxide. The ethylene oxide leaves the top of the stripper as a concentrated aqueous ethylene oxide stream.
In one well-known process, ethylene oxide is then catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol compared to the known process wherein ethylene oxide is reacted with a large excess of water to form ethylene glycol in a non-catalytic process.
Such a process for the production of ethylene glycol via ethylene carbonate can be run in series with the process for the production of ethylene oxide. That is, the ethylene oxide is not purified after absorption and stripping, but is supplied to a carboxylation reactor as the concentrated aqueous stream from the top of the stripper.
During the start-up of such a combined process, the ethylene oxide to ethylene glycol section of the plant is prepared to receive an aqueous ethylene oxide stream before such a stream is actually provided. To this end, this section of the plant is heated up, pressurized and several internal recycles, such as catalyst, CO2 and internal liquid recycles are established. A water stream is supplied to the carboxylation reactor and is removed from the process, further downstream, via a dehydrator.
Problems may occur in the ethylene oxide to ethylene glycol part of the process at a stage where little or no aqueous ethylene oxide is being produced in the ethylene to ethylene oxide part of the process. At this stage, the feed streams being supplied to the process, and passing through the carboxylation and hydrolysis stages, will be a concentrated catalyst stream, a carbon dioxide stream and water. When only these feeds are passed through the process, instead of the desired concentrated solution of catalyst in a catalyst separation section and dilute solutions of catalysts in the rest of the ethylene oxide to ethylene glycol section of the plant, a homogenous distribution of the catalyst solution throughout the entire ethylene oxide to ethylene glycol section of the plant will result. Alternatively, certain sections of the plant (e.g. the catalyst separation section) must be bypassed until an aqueous ethylene oxide stream is being provided. These sections must then be started-up separately, at a later stage, adding to the complexity of the overall start-up process.
The present inventors have sought to provide an improved start-up procedure for this process for the production of an alkylene glycol from an alkene allowing avoidance of the above problems and an overall increase in efficiency of the process.